Thread: Zx81
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Old 15th Mar 2020, 2:50 pm   #73
Join Date: May 2008
Location: Derby, UK.
Posts: 7,732
Default Re: Zx81

The ZX81 supply is positive tip, and -- by happy accident or cunning design -- will not cause any lasting damage if plugged into the EAR or MIC socket for long enough to notice something is amiss. (Applying negative voltages to inputs of MOS ICs with protection diodes, even when unpowered, is a bad idea. Sinclair would probably have known that when choosing the wiring and reckoned accordingly, selecting the option least likely to be damaged by misconnection without adding expensive protection diodes, resistors or fuses!)

The Spectrum supply used a "barrel" connector with negative inside, so no possibility of misconnection. I don't know if the change was for better reliability (only one sprung contact) or user-proofing.
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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